This invention relates, in general, to mold assemblies, and more particularly to a mold assembly wherein a molded carrier ring cavity and a device cavity are filled with different encapsulating materials.
Semiconductor manufacturers have used molding assemblies to encapsulate semiconductor devices for some time. Encapsulation or packaging serves to protect both the semiconductor device and the fragile signal interconnects from possible damage caused by interaction with elements in the environment. Further, some encapsulation techniques have included a molded carrier ring to protect the package during subsequent handling and testing.
Typically, a molding assembly has a plurality of cavities adapted to receive a semiconductor device. Cavities have been configured such that an annular cavity surrounds and is an extension of an inner cavity. The inner cavity receives the semiconductor device while the annular cavity serves to form the molded carrier ring. The molding assembly also has a receptacle, commonly called a pot, for receiving the encapsulating material. Some molding assemblies have more than one pot. The pot and cavity are connected by a runner through which encapsulating material flows.
In general, encapsulating material for semiconductor devices is a thermoset plastic which is inserted into the mold pot in a pelletized form. Under the influence of heat the pellet fuses, thereby attaining a liquid state. A ram or plunger forces fluidized encapsulating material to flow from the pot, through the runners, into the cavities; filling the annular cavity first followed by the inner cavity. Ideally, the encapsulating material remains in a liquid form until both inner and annular cavities are filled. Then, the encapsulating material returns to a solid form, a process commonly referred to as curing. However, it is known that curing begins during transfer of encapsulating material into the cavities, hence minimizing the transfer time is critical.
As an example, optimum encapsulation entails completion of fluidization and transfer of encapsulating material from pots to cavities before the material begins to cure. As the encapsulating material cures it becomes less fluidized, hence more viscous. The increased viscosity may cause encapsulating material to dislodge thin connecting wires on the semiconductor device upon entry into the inner cavity; a phenomenon commonly called wire sweep. Since fluidized encapsulating material is believed to cure as it flows toward the inner cavity, the distance from pots to cavities is critical, and generally has been minimized. Another cause of wire sweep may be that encapsulating material is injected under too high a pressure. Furthermore, lack of fluidization of encapsulating material may cause voids, which also compromises the integrity of the encapsulation. Multipot molds have been used in an attempt to overcome some of the problems of a single-pot mold; however, very high pressures are required in order to obtain good encapsulation. Accordingly, it would be beneficial to have a mold pot-cavity configuration that minimizes the length of runners as well as optimizes the expense of encapsulating material.